There are two ways to create an SSH tunnel, local and remote port
forwarding (there’s also dynamic forwarding, but we won’t cover that
here). The best way to understand these is by an example, let’s start
with local port forwarding.

Imagine you’re on a private network which doesn’t allow connections to a
specific server. Let’s say you’re at work and imgur.com is being
blocked. To get around this we can create a tunnel through a server
which isn’t on our network and thus can access Imgur.

$ ssh -L 9000:imgur.com:80 user@example.com

The key here is -L which says we’re doing local port forwarding. Then
it says we’re forwarding our local port 9000 to imgur.com:80,
which is the default port for HTTP. Now open your browser and go to
http://localhost:9000.

The awesome thing about SSH tunnels is that they are encrypted. Nobody
is going to see what sites you’re visiting, they’ll only see an SSH
connection to your server.

Connecting to a database behind a firewall

Another good example is if you need to access a port on your server
which can only be accessed from localhost and not remotely.

An example here is when you need to connect to a database console, which
only allows local connection for security reasons. Let’s say you’re
running PostgreSQL on your server, which by default listens on the port
5432.

$ ssh -L 9000:localhost:5432 user@example.com

The part that changed here is the localhost:5432, which says to
forward connections from your local port 9000 to localhost:5432 on
your server. Now we can simply connect to our database.

$ psql -h localhost -p 9000

Now let’s stop here for a little bit an explain what is actually going
on. In the first example the 9000:imgur.com:80 is actually saying
forward my local port 9000 to imgur.com at port 80. You can imagine
SSH on your server actually making a connection (a tunnel) between those
two ports, one on your local machine, and one on the target destination.

If we however say something like 9000:localhost:5432, it means
localhost from the server’s perspective, not localhost on your
machine. This means forward my local port 9000 to port 5432 on
the server, because when you’re on the server, localhost means the
server itself.

This might be a bit confusing, but it is important to understand what
the syntax actually means here.

Remote port forwarding

Now comes the second part of this tutorial, which is remote port
forwarding. This is again best to explain with an example.

Say that you’re developing a Rails application on your local machine,
and you’d like to show it to a friend. Unfortunately your ISP didn’t
provide you with a public IP address, so it’s not possible to connect
to your machine directly via the internet.

Sometimes this can be solved by configuring NAT (Network Address
Translation) on your router, but this doesn’t always work, and it
requires you to change the configuration on your router, which isn’t
always desirable. This solution also doesn’t work when you don’t have
admin access on your network.

To fix this problem you need to have another computer, which is publicly
accessible and have SSH access to it. It can be any server on the
internet, as long as you can connect to it. We’ll tell SSH to make a
tunnel that opens up a new port on the server, and connects it to a
local port on your machine.

$ ssh -R 3000:localhost:9000 user@example.com

The syntax here is very similar to local port forwarding, with a
single change of -L for -R. But as with local port forwarding, the
syntax remains the same.

First you need to specify a local port on your machine which you wish
to expose publicly, which in this case is 3000. Next there is the
address on which the server should listen to, and a port on which the
server should listen to, which in this case is 9000.

There is one more thing you need to do to enable this. SSH doesn’t by
default allow remote hosts to forwarded ports. To enable this open
/etc/ssh/sshd_config and add the following line somewhere in that
config file.

GatewayPorts yes

Make sure you add it only once!

$ sudo vim /etc/ssh/sshd_config

And restart SSH

$ sudo service ssh restart

After this you should be able to connect to the server remotely, even
from your local machine. The way this would work is that you would
first create an SSH tunnel that forwards traffic from the server on
port 9000 to your local machine on port 3000. This means that if
you connect to the server on port 9000 from your local machine,
you’ll actually make a request to your machine through the SSH tunnel.

A few closing tips

You might have noticed that every time we create a tunnel you also SSH
into the server and get a shell. This isn’t usually necessary, as you’re
just trying to create a tunnel. To avoid this we can run SSH with the
-nNT flags, such as the following, which will cause SSH to not
allocate a tty and only do the port forwarding.

$ ssh -nNT -L 9000:imgur.com:80 user@example.com

SSH has a huge number of features, so I’d recommend you to checkout the
manual page at man ssh, which contains even more tips.